Monday, August 19, 2013

Everything you need to know about the hyperloop in 25 words or less

The reason it won't work is that the tube has to be 1) straight (so that a capsule can move through it safely at 700 miles per hour) 2) sealed (so that a partial vacuum can be maintained) and 3) hundreds of miles long (so that you can travel useful distances). A tube that is is straight, sealed, and hundreds of miles long will undergo thermal expansion that will amount to hundreds of feet of movement at either end. Until and unless this problem is solved, the hyperloop won't fly. (This is far from the only show-stopper, but it's the easiest one to explain.)

15 comments:

Points 2+: I don't see why it has to be "straight" as opposed to minimal and gradual curves and no sharp ones. I don't see why segments couldn't be shorter with the option of disembarking or seamlessly transferring to the next. A bit of math should give us some ballpark figures of feasible segment lengths, a function of acceleration/deceleration requirements at minimum...

Surely sealed expansion joints are a minor engineering issue..?

I'm not taking a real position on the practicality of the idea, but I'd say this post is a bit heavy on the "grumpy old man" sauce! :)

107. but who's counting? I hope you'll grant me a little poetic license in the titles of my posts.

> I don't see why it has to be "straight" as opposed to minimal and gradual curves

Of course it doesn't have to be perfectly straight. The maximum curvature is determined by the g-forces that people can tolerate without getting motion sick. At 700 MPH that's a pretty damn gradual curve. I don't remember the exact numbers, but they're not hard to find, or to calculate yourself. I think the maximum force is about 0.4g. For the purposes of thermal expansion, it's essentially straight. To make a thermal expansion joint you either need to have two pieces physically move past one another, which is all but impossible if you want to maintain an airtight seal, or you need a very sharp bend, much sharper than is feasible in this case.

> Surely sealed expansion joints are a minor engineering issue..?

Alas no. Look up photos of the Alaska pipeline, for example. You'll see that it zigzags all over the place. This is to accommodate thermal expansion.

Also, the cost estimates are based on the assumption that the segments are welded together. You might be able to come up with some kind of thermal expansion joint that wold hold a vacuum, but coming up with one that doesn't blow the budget would require a major breakthrough. If you want to make claims about cost-effectiveness (and Musk does) then you can't hand-wave this problem away.

I'm sorry if I sound like a grumpy old man. I would really like the hyperloop to succeed. I think it would be wicked cool. And I admire Musk for going as far as he has with his design and analysis. But the thermal expansion problem looks like a show stopper to me. Alas.

re: sealed expansion joints - I'm still sceptical that one couldn't say make use of some principles on hydraulics and design some tubes that nest with flexible seals that could handle extremely low pressures...BUT

I'm not a structural engineer of any kind so have no real suggestion, and on the other hand I don't like buying in to the whole blind faith in the magic of technology thing either so I would give my opinion on the matter an appropriately low weight.

Tell you what, if it gets up and running, you can buy my first ticket, and if not, I'll buy yours :D

> I'm still sceptical that one couldn't say make use of some principles on hydraulics and design some tubes that nest with flexible seals that could handle extremely low pressures

There is no doubt you could do this. You could, for example, flare one end of a segment, stick the next segment inside, and seal it with an O-ring. But doing it is not enough. You have to do it at a cost that comparable to welding, otherwise your economic model becomes invalid. THAT is the unsolved problem.

By building a system on pylons, where the tube is not rigidly fixed at any point, you can dramatically mitigate Earthquake risk and avoid the need for expansion joints.

Page 27, section 4.2.3:

The tube will be supported by pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion as well as dampened lateral slip to reduce the risk posed by earthquakes. In addition, the pillar to tube connection nominal position will be adjustable vertically and laterally to ensure proper alignment despite possible ground settling. These minimally constrained pillars to tube joints will also allow a smoother ride. Specially designed slip joints at each stations will be able take any tube length variance due to thermal expansion. This is an ideal location for the thermal expansion joints as the speed is much lower nearby the stations. It thus allows the tube to be smooth and welded along the high speed gliding middle section.

The funny thing is that the Dr. Drang actually quoted the first part, and last few sentences of the second part. He managed to miss "not rigidly fixed at any point" part and assumed the tube is attached rigidly in his pictures.

The sad thing is Hacker News thread. I tried to raise this objection, but my comment was way below silly discussion about Dr. Drang tone and Musk authority...

You're missing the point: if the tube is (effectively) straight and welded. then the thermal expansion at the ends will be cumulative, and amount to hundreds of feet of movement. We're talking football fields not millimeters.

"By building a system on pylons, where the tube is not rigidly fixed at any point, you can dramatically mitigate Earthquake risk and avoid the need for expansion joints. Tucked away inside each pylon, you could place two adjustable lateral (XY) dampers and one vertical (Z) damper.

These would absorb the SMALL length changes between pylons due to thermal changes..."

[Emphasis added]

The game changes radically when the length changes are not small. Which they aren't.

Yes, it says so. It could be a typo, or just part of the whole truth. E.g. it could mean that dampers would absorb small part of thermal change. The much larger part would be absorbed by the movement of non-rigidly fixed tupe.

All of this is the first part of the paper for which Musk "apologize[s] in advance for my loose use of language and imperfect analogies" (page 1). But even there Musk is clearly aware of large length changes. Just a sentence later:

"A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube."

The second part of the paper is more precise. I've already quoted relevant (section 4.2.3) part of the second section in the comment above. Do you want me to repeat it here?

By the way, do you accept the tube could freely move in longitudal direction? If yes, I don't see any difference between slipping for a few millimeters along the longitude vs hundreds of feet. Thermal expansion wouldn't happen in an instance, would it?

I'm not saying he's not aware of this, I'm saying he hasn't thought it through.

> do you accept the tube could freely move in longitudal direction?

In principle, yes, of course. But the devil is in the details. It's a lot easier to imagine a mount that would allow a tube to move a few millimeters than one that would allow movement of hundreds of feet. A few millimeters could probably be absorbed by a chunk of rubber. Anything more than a couple of centimeters would (probably) require some kind of bearing. Big, big (BIG!) cost difference.

No artifact with the kind of thermal expansion characteristic of the hyperloop has ever been constructed. There may well be a solution to the thermal expansion problem. There may even be a cost-effective solution to the thermal expansion problem. But there is no currently *known* cost-effective solution to the thermal expansion problem AFAICT. Until that changes, the hyperloop is a research program, not a serious technology proposal.

I agree, Musk is not detailed enough here. Some kind of linear bearing is certainly required. I know nearly nothing about them (do you?), but plain (no balls) bearing would probably work. Intuitively, designing very-low-speed, low-precision linear bearing is much easier and cheaper than designing airtight slip joints while preserving smooth inside surface. But I would love someone knowledgable to correct me.

In fact, your first point - the tube must be straight - is what allows free-longitudal-movement design. Alaska pipeline couldn't afford to be that straight.

Your original post is reasonable, except for "It won't work" link to the badly researched article that doesn't really support your claim. The author clearly didn't understand that the tube would freely slip in linear direction. He criticized the wrong part.